BDD style scenarios are meant to be completely independent. In this case, you would have 2 scenarios:
Scenario: I can check out with an updated order quantity
Given I login
And I search
And I add to the cart
And I update the quantity
When I checkout
Then I get a confirmation email (or whatever)
Scenario: I can purchase items
Given I login
And I search
And I add to the cart
And I update the quantity
When I checkout
Then I get a confirmation email (or whatever)
If there's a concern about each of those steps actually being several steps and that looking really ugly in the scenario with 20 givens, so long as each of those actions is tested separately, you can use a compound step. That's a step that, in its definition, calls other steps (through code, not Gherkin). You take the same actions, but the Gherkin has far fewer entries.
The important part is that no scenario should require action from a previous scenario in BDD.
To further clarify, there is a way to re-order things if you really must, but it's very bad practice. If you choose to go that road, see this question:
How do i execute story files in specific order in serenity BDD Jbehave